Multi-Level Compound Prediction

ABSTRACT

Coding a current block using multi-level compound predictor is disclosed. A method includes generating a first compound predictor by combining at least two predictor blocks for the current block, generating a second predictor for the current block, and combining the first compound predictor and the second predictor to obtain a prediction block for encoding or decoding the current block. An indicator may be sent from the encoder to the decoder to indicate when multi-level compound prediction is used to encode a block.

BACKGROUND

Digital video streams may represent video using a sequence of frames orstill images. Digital video can be used for various applicationsincluding, for example, video conferencing, high definition videoentertainment, video advertisements, or sharing of user-generatedvideos. A digital video stream can contain a large amount of data andconsume a significant amount of computing or communication resources ofa computing device for processing, transmission or storage of the videodata. Various approaches have been proposed to reduce the amount of datain video streams, including compression and other encoding techniques.

Encoding based on motion estimation and compensation may be performed bybreaking frames or images into blocks that are predicted based on one ormore prediction blocks of reference frames. Differences (i.e., residualerrors) between blocks and prediction blocks are compressed and encodedin a bitstream. A decoder uses the differences and the reference framesto reconstruct the frames or images.

SUMMARY

Disclosed herein are aspects, features, elements, and implementationsfor encoding and decoding blocks using multi-level compound prediction.

One aspect of the disclosed implementations is a method for coding acurrent block. The method includes generating a first compound predictorby combining at least two predictor blocks for the current block,generating a second predictor for the current block, and combining thefirst compound predictor and the second predictor to obtain a predictionblock for coding the current block. Coding the current block can includeencoding or decoding the current block.

Another aspect is an apparatus, including a memory and a processor, forencoding a current block according to one implementation of thisdisclosure. The processor is configured to execute instructions storedin the memory to generate a first compound predictor by combining atleast two predictor blocks for the current block, generate a secondpredictor for the current block, and combine the first compoundpredictor and the second predictor to obtain a prediction block forencoding the current block.

Another aspect is an apparatus, including a memory and a processor, fordecoding a current block according to one implementation. The processoris configured to execute instructions stored in the memory to decode,from an encoded bitstream, an indicator that identifies that multi-levelcompound prediction was used to encode the current block, generate afirst compound predictor by combining at least two predictor blocks forthe current block, generate a second predictor for the current block,and combine the first compound predictor and the second predictor toobtain a prediction block for decoding the current block.

These and other aspects of the present disclosure are disclosed in thefollowing detailed description of the embodiments, the appended claimsand the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views.

FIG. 1 is a schematic of a video encoding and decoding system.

FIG. 2 is a block diagram of an example of a computing device that canimplement a transmitting station or a receiving station.

FIG. 3 is a diagram of a video stream to be encoded and subsequentlydecoded.

FIG. 4 is a block diagram of an encoder according to implementations ofthis disclosure.

FIG. 5 is a block diagram of a decoder according to implementations ofthis disclosure.

FIG. 6 is a flowchart diagram of a process for encoding a current blockusing multi-level compound prediction according to an implementation ofthis disclosure.

FIG. 7 is a diagram of an inter tri-prediction according toimplementations of this disclosure.

FIG. 8 is a diagram of an example of a reference frame buffer accordingto implementations of this disclosure.

FIG. 9 is a diagram of an inter-intra tri-prediction according toimplementations of this disclosure.

FIG. 10 is a diagram of an inter quad-prediction according toimplementations of this disclosure.

FIG. 11 is a diagram of an inter-intra quad-prediction according toimplementations of this disclosure.

FIG. 12 is a diagram of a multi-level prediction according toimplementations of this disclosure.

FIG. 13 is a flowchart diagram of a process for decoding a current blockaccording to an implementation of this disclosure.

DETAILED DESCRIPTION

As mentioned above, compression schemes related to coding video streamsmay include breaking images into blocks and generating a digital videooutput bitstream (i.e., an encoded bitstream) using one or moretechniques to limit the information included in the output bitstream. Areceived bitstream can be decoded to re-create the blocks and the sourceimages from the limited information. Encoding a video stream, or aportion thereof, such as a frame or a block, can include using temporalor spatial similarities in the video stream to improve codingefficiency. For example, a current block of a video stream may beencoded based on identifying a difference (residual) between thepreviously coded pixel values, or between a combination of previouslycoded pixel values, and those in the current block.

Encoding using spatial similarities can be known as intra prediction.Intra prediction can attempt to predict the pixel values of a block of aframe of a video stream using pixels peripheral to the block; that is,using pixels that are in the same frame as the block but that areoutside the block. A prediction block resulting from intra prediction isreferred to herein as an intra predictor. Intra prediction can beperformed along a direction of prediction where each direction cancorrespond to an intra prediction mode. The intra prediction mode can besignalled by an encoder to a decoder.

Encoding using temporal similarities can be known as inter prediction.Inter prediction can attempt to predict the pixel values of a blockusing a possibly displaced block or blocks from a temporally nearbyframe (i.e., reference frame) or frames. A temporally nearby frame is aframe that appears earlier or later in time in the video stream than theframe of the block being encoded. A prediction block resulting frominter prediction is referred to herein as inter predictor.

Inter prediction is performed using a motion vector. A motion vectorused to generate a prediction block refers to a frame other than acurrent frame, i.e., a reference frame. Reference frames can be locatedbefore or after the current frame in the sequence of the video stream.Some codecs use up to eight reference frames, which can be stored in aframe buffer. The motion vector can refer to (i.e., use) one of thereference frames of the frame buffer.

Two predictor blocks can be combined to form a compound predictor for ablock or region of a video image. A compound predictor can be created bycombining two or more predictors determined using, for example, theaforementioned prediction methods (i.e., inter and/or intra prediction).For example, a compound predictor can be combination of a firstpredictor and a second predictor which can be two intra predictors(i.e., intra+intra), an intra predictor and an inter predictor (i.e.,intra+inter) or two inter predictors (i.e., inter+inter). For example,in the case of inter+inter, compound inter prediction can employ a firstmotion vector to obtain a predictor from a first reference frame, and asecond motion vector to obtain a predictor from a second referenceframe. The reference frames can both be in the past, both in the future,or some combination thereof. The second motion vector can be independentof, or derived from, the first motion vector. As another example, and inthe case of intra-inter, compound prediction can employ a firstpredictor generated by an intra prediction operation and a secondpredictor generated by an inter prediction operation.

In forming the compound predictor, an encoder can perform averaging, aweighted combination, a filtering operation, or a more complex form ofestimating the significance of the value of each predictor, e.g., on aper-pixel basis to generate pixel values for the combined predictorusing pixels of the two individual predictors. Accordingly, combiningpredictors can result in a compound predictor that can be an average ofthe two values or a weighted average that estimates the significance ofthe value of each predictor, for example.

A motion vector can be selected from a reference motion vector list ofcandidate reference motion vectors. The candidate reference motionvectors can include motion vectors from any previously coded (ordecoded) blocks in the video stream, such as a block from a previouslycoded (or decoded) frame, or a block from the same frame that has beenpreviously encoded (or decoded). The candidate reference motion vectorscan be obtained from a co-located block (of the current block) and itssurrounding blocks in a reference frame. For example, the surroundingblocks can include a block to the right, bottom-left, bottom-right of,or below the co-located block. In the case of non-compound interprediction (i.e., single inter prediction), the candidate referencemotion vectors for a block can include, for at least some referenceframes, a single predictor motion vector per reference frame that can bethe best motion vector for that reference frame. In the case of compoundinter prediction, pairs of reference frames can be evaluated todetermine a best motion vector(s) per pair of reference frames. The bestmotion vector(s) per pair can be included in a reference motion vectorlist.

In either case (i.e., intra, inter, or compound prediction), aprediction block is generated and can be subtracted from the block to beencoded to form the residual block representing the difference betweenthe blocks.

Implementations of this disclosure can improve motion prediction viamulti-level compound prediction. Using multi-level compound prediction,new kinds of compound predictors can be created. Multi-level compoundprediction provides more candidate predictors for the coding of a blockor a region of video. For example, motion prediction using multi-levelcompound prediction can better adapt to the specific motion and contentvarieties of the block to be encoded, resulting in compressionimprovements. Whereas some video codecs may use a maximum of two of thereference frames of the reference frame buffer, multi-level compoundprediction can more effectively leverage the reference frames toidentify an improved prediction block by generating additional candidatereference motion vectors using additional combinations of the referenceframes. Compression performance can be improved while a reasonable levelof encoder and decoder complexity is maintained.

Details are described herein after first describing an environment inwhich the multi-level compound prediction disclosed herein may beimplemented.

FIG. 1 is a schematic of a video encoding and decoding system 100. Atransmitting station 102 can be, for example, a computer having aninternal configuration of hardware such as that described in FIG. 2.However, other suitable implementations of the transmitting station 102are possible. For example, the processing of the transmitting station102 can be distributed among multiple devices.

A network 104 can connect the transmitting station 102 and a receivingstation 106 for encoding and decoding of the video stream. Specifically,the video stream can be encoded in the transmitting station 102 and theencoded video stream can be decoded in the receiving station 106. Thenetwork 104 can be, for example, the Internet. The network 104 can alsobe a local area network (LAN), wide area network (WAN), virtual privatenetwork (VPN), cellular telephone network or any other means oftransferring the video stream from the transmitting station 102 to, inthis example, the receiving station 106.

The receiving station 106, in one example, can be a computer having aninternal configuration of hardware such as that described in FIG. 2.However, other suitable implementations of the receiving station 106 arepossible. For example, the processing of the receiving station 106 canbe distributed among multiple devices.

Other implementations of the video encoding and decoding system 100 arepossible. For example, an implementation can omit the network 104. Inanother implementation, a video stream can be encoded and then storedfor transmission at a later time to the receiving station 106 or anyother device having memory. In one implementation, the receiving station106 receives (e.g., via the network 104, a computer bus, and/or somecommunication pathway) the encoded video stream and stores the videostream for later decoding. In an example implementation, a real-timetransport protocol (RTP) is used for transmission of the encoded videoover the network 104. In another implementation, a transport protocolother than RTP may be used, e.g., a Hyper-Text Transfer Protocol(HTTP)-based video streaming protocol.

When used in a video conferencing system, for example, the transmittingstation 102 and/or the receiving station 106 may include the ability toboth encode and decode a video stream as described below. For example,the receiving station 106 could be a video conference participant whoreceives an encoded video bitstream from a video conference server(e.g., the transmitting station 102) to decode and view and furtherencodes and transmits its own video bitstream to the video conferenceserver for decoding and viewing by other participants.

FIG. 2 is a block diagram of an example of a computing device 200 thatcan implement a transmitting station or a receiving station. Forexample, the computing device 200 can implement one or both of thetransmitting station 102 and the receiving station 106 of FIG. 1. Thecomputing device 200 can be in the form of a computing system includingmultiple computing devices, or in the form of a single computing device,for example, a mobile phone, a tablet computer, a laptop computer, anotebook computer, a desktop computer, and the like.

A CPU 202 in the computing device 200 can be a central processing unit.Alternatively, the CPU 202 can be any other type of device, or multipledevices, capable of manipulating or processing information now-existingor hereafter developed. Although the disclosed implementations can bepracticed with a single processor as shown, e.g., the CPU 202,advantages in speed and efficiency can be achieved using more than oneprocessor.

A memory 204 in the computing device 200 can be a read-only memory (ROM)device or a random access memory (RAM) device in an implementation. Anyother suitable type of storage device can be used as the memory 204. Thememory 204 can include code and data 206 that is accessed by the CPU 202using a bus 212. The memory 204 can further include an operating system208 and application programs 210, the application programs 210 includingat least one program that permits the CPU 202 to perform the methodsdescribed here. For example, the application programs 210 can includeapplications 1 through N, which further include a video codingapplication that performs the methods described here. The computingdevice 200 can also include a secondary storage 214, which can, forexample, be a memory card used with a computing device 200 that ismobile. Because the video communication sessions may contain asignificant amount of information, they can be stored in whole or inpart in the secondary storage 214 and loaded into the memory 204 asneeded for processing.

The computing device 200 can also include one or more output devices,such as a display 218. The display 218 may be, in one example, a touchsensitive display that combines a display with a touch sensitive elementthat is operable to sense touch inputs. The display 218 can be coupledto the CPU 202 via the bus 212. Other output devices that permit a userto program or otherwise use the computing device 200 can be provided inaddition to or as an alternative to the display 218. When the outputdevice is or includes a display, the display can be implemented invarious ways, including by a liquid crystal display (LCD), a cathode-raytube (CRT) display or light emitting diode (LED) display, such as anorganic LED (OLED) display.

The computing device 200 can also include or be in communication with animage-sensing device 220, for example a camera, or any otherimage-sensing device 220 now existing or hereafter developed that cansense an image such as the image of a user operating the computingdevice 200. The image-sensing device 220 can be positioned such that itis directed toward the user operating the computing device 200. In anexample, the position and optical axis of the image-sensing device 220can be configured such that the field of vision includes an area that isdirectly adjacent to the display 218 and from which the display 218 isvisible.

The computing device 200 can also include or be in communication with asound-sensing device 222, for example a microphone, or any othersound-sensing device now existing or hereafter developed that can sensesounds near the computing device 200. The sound-sensing device 222 canbe positioned such that it is directed toward the user operating thecomputing device 200 and can be configured to receive sounds, forexample, speech or other utterances, made by the user while the useroperates the computing device 200.

Although FIG. 2 depicts the CPU 202 and the memory 204 of the computingdevice 200 as being integrated into a single unit, other configurationscan be utilized. The operations of the CPU 202 can be distributed acrossmultiple machines (each machine having one or more processors) that canbe coupled directly or across a local area or other network. The memory204 can be distributed across multiple machines such as a network-basedmemory or memory in multiple machines performing the operations of thecomputing device 200. Although depicted here as a single bus, the bus212 of the computing device 200 can be composed of multiple buses.Further, the secondary storage 214 can be directly coupled to the othercomponents of the computing device 200 or can be accessed via a networkand can comprise a single integrated unit such as a memory card ormultiple units such as multiple memory cards. The computing device 200can thus be implemented in a wide variety of configurations.

FIG. 3 is a diagram of an example of a video stream 300 to be encodedand subsequently decoded. The video stream 300 includes a video sequence302. At the next level, the video sequence 302 includes a number ofadjacent frames 304. While three frames are depicted as the adjacentframes 304, the video sequence 302 can include any number of adjacentframes 304. The adjacent frames 304 can then be further subdivided intoindividual frames, e.g., a frame 306. At the next level, the frame 306can be divided into a series of segments 308 or planes. The segments 308can be subsets of frames that permit parallel processing, for example.The segments 308 can also be subsets of frames that can separate thevideo data into separate colors. For example, the frame 306 of colorvideo data can include a luminance plane and two chrominance planes. Thesegments 308 may be sampled at different resolutions.

Whether or not the frame 306 is divided into the segments 308, the frame306 may be further subdivided into blocks 310, which can contain datacorresponding to, for example, 16×16 pixels in the frame 306. The blocks310 can also be arranged to include data from one or more segments 308of pixel data. The blocks 310 can also be of any other suitable sizesuch as 4×4 pixels, 8×8 pixels, 16×8 pixels, 8×16 pixels, 16×16 pixelsor larger.

FIG. 4 is a block diagram of an encoder 400 according to implementationsof this disclosure. The encoder 400 can be implemented, as describedabove, in the transmitting station 102 such as by providing a computersoftware program stored in memory, for example, the memory 204. Thecomputer software program can include machine instructions that, whenexecuted by a processor such as the CPU 202, cause the transmittingstation 102 to encode video data in the manner described herein. Theencoder 400 can also be implemented as specialized hardware included in,for example, the transmitting station 102. The encoder 400 has thefollowing stages to perform the various functions in a forward path(shown by the solid connection lines) to produce an encoded orcompressed bitstream 420 using the video stream 300 as input: anintra/inter prediction stage 402, a transform stage 404, a quantizationstage 406, and an entropy encoding stage 408. The encoder 400 may alsoinclude a reconstruction path (shown by the dotted connection lines) toreconstruct a frame for encoding of future blocks. In FIG. 4, theencoder 400 has the following stages to perform the various functions inthe reconstruction path: a dequantization stage 410, an inversetransform stage 412, a reconstruction stage 414, and a loop filteringstage 416. Other structural variations of the encoder 400 can be used toencode the video stream 300.

When the video stream 300 is presented for encoding, the frame 306 canbe processed in units of blocks. At the intra/inter prediction stage402, a block can be encoded using intra-frame prediction (also calledintra-prediction) or inter-frame prediction (also calledinter-prediction), or a combination of both. In any case, a predictionblock can be formed. In the case of intra-prediction, all or a part of aprediction block may be formed from samples in the current frame thathave been previously encoded and reconstructed. In the case ofinter-prediction, all or part of a prediction block may be formed fromsamples in one or more previously constructed reference framesdetermined using motion vectors.

Next, still referring to FIG. 4, the prediction block can be subtractedfrom the current block at the intra/inter prediction stage 402 toproduce a residual block (also called a residual). The transform stage404 transforms the residual into transform coefficients in, for example,the frequency domain using block-based transforms. Such block-basedtransforms include, for example, the Discrete Cosine Transform (DCT) andthe Asymmetric Discrete Sine Transform (ADST). Other block-basedtransforms are possible. Further, combinations of different transformsmay be applied to a single residual. In one example of application of atransform, the DCT transforms the residual block into the frequencydomain where the transform coefficient values are based on spatialfrequency. The lowest frequency (DC) coefficient at the top-left of thematrix and the highest frequency coefficient at the bottom-right of thematrix. It is worth noting that the size of a prediction block, andhence the resulting residual block, may be different from the size ofthe transform block. For example, the prediction block may be split intosmaller blocks to which separate transforms are applied.

The quantization stage 406 converts the transform coefficients intodiscrete quantum values, which are referred to as quantized transformcoefficients, using a quantizer value or a quantization level. Forexample, the transform coefficients may be divided by the quantizervalue and truncated. The quantized transform coefficients are thenentropy encoded by the entropy encoding stage 408. Entropy coding may beperformed using any number of techniques, including token and binarytrees. The entropy-encoded coefficients, together with other informationused to decode the block, which may include for example the type ofprediction used, transform type, motion vectors and quantizer value, arethen output to the compressed bitstream 420. The information to decodethe block may be entropy coded into block, frame, slice and/or sectionheaders within the compressed bitstream 420. The compressed bitstream420 can also be referred to as an encoded video stream or encoded videobitstream, and the terms will be used interchangeably herein.

The reconstruction path in FIG. 4 (shown by the dotted connection lines)can be used to ensure that both the encoder 400 and a decoder 500(described below) use the same reference frames and blocks to decode thecompressed bitstream 420. The reconstruction path performs functionsthat are similar to functions that take place during the decodingprocess that are discussed in more detail below, including dequantizingthe quantized transform coefficients at the dequantization stage 410 andinverse transforming the dequantized transform coefficients at theinverse transform stage 412 to produce a derivative residual block (alsocalled a derivative residual). At the reconstruction stage 414, theprediction block that was predicted at the intra/inter prediction stage402 can be added to the derivative residual to create a reconstructedblock. The loop filtering stage 416 can be applied to the reconstructedblock to reduce distortion such as blocking artifacts.

Other variations of the encoder 400 can be used to encode the compressedbitstream 420. For example, a non-transform based encoder 400 canquantize the residual signal directly without the transform stage 404for certain blocks or frames. In another implementation, an encoder 400can have the quantization stage 406 and the dequantization stage 410combined into a single stage.

FIG. 5 is a block diagram of a decoder 500 according to implementationsof this disclosure. The decoder 500 can be implemented in the receivingstation 106, for example, by providing a computer software programstored in the memory 204. The computer software program can includemachine instructions that, when executed by a processor such as the CPU202, cause the receiving station 106 to decode video data in the mannerdescribed herein. The decoder 500 can also be implemented in hardwareincluded in, for example, the transmitting station 102 or the receivingstation 106. The decoder 500, similar to the reconstruction path of theencoder 400 discussed above, includes in one example the followingstages to perform various functions to produce an output video stream516 from the compressed bitstream 420: an entropy decoding stage 502, adequantization stage 504, an inverse transform stage 506, anintra/inter-prediction stage 508, a reconstruction stage 510, a loopfiltering stage 512 and a deblocking filtering stage 514. Otherstructural variations of the decoder 500 can be used to decode thecompressed bitstream 420.

When the compressed bitstream 420 is presented for decoding, the dataelements within the compressed bitstream 420 can be decoded by theentropy decoding stage 502 to produce a set of quantized transformcoefficients. The dequantization stage 504 dequantizes the quantizedtransform coefficients (e.g., by multiplying the quantized transformcoefficients by the quantizer value), and the inverse transform stage506 inverse transforms the dequantized transform coefficients using theselected transform type to produce a derivative residual that can beidentical to that created by the inverse transform stage 412 in theencoder 400. Using header information decoded from the compressedbitstream 420, the decoder 500 can use the intra/inter-prediction stage508 to create the same prediction block as was created in the encoder400, e.g., at the intra/inter prediction stage 402. At thereconstruction stage 510, the prediction block can be added to thederivative residual to create a reconstructed block. The loop filteringstage 512 can be applied to the reconstructed block to reduce blockingartifacts. Other filtering can be applied to the reconstructed block. Inthis example, the deblocking filtering stage 514 is applied to thereconstructed block to reduce blocking distortion, and the result isoutput as an output video stream 516. The output video stream 516 canalso be referred to as a decoded video stream, and the terms will beused interchangeably herein.

Other variations of the decoder 500 can be used to decode the compressedbitstream 420. For example, the decoder 500 can produce the output videostream 516 without the deblocking filtering stage 514. In someimplementations of the decoder 500, the deblocking filtering stage 514is applied before the loop filtering stage 512. Additionally, oralternatively, the encoder 400 includes a deblocking filtering stage inaddition to the loop filtering stage 416.

FIG. 6 is a flowchart diagram of a process 600 for encoding a currentblock using multi-level compound prediction according to animplementation of this disclosure. The process 600 can be implemented inan encoder such as the encoder 400 of FIG. 4.

The process 600 can be implemented, for example, as a software programthat can be executed by computing devices such as transmitting station102. The software program can include machine-readable instructions thatcan be stored in a memory such as the memory 204 or the secondarystorage 214, and that can be executed by a processor, such as CPU 202,to cause the computing device to perform the process 600. In at leastsome implementations, the process 600 can be performed in whole or inpart by the intra/inter prediction stage 402 of the encoder 400 of FIG.4.

The process 600 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 600 can bedistributed using different processors, memories, or both. Use of theterms “processor” or “memory” in the singular encompasses computingdevices that have one processor or one memory as well as devices thathave multiple processors or multiple memories that can be used in theperformance of some or all of the recited steps.

At 602, the process 600 generates a first compound predictor bycombining at least two predictor blocks for the current block. Thepredictor blocks that form a compound predictor may be referred toherein as constituent predictors or constituent blocks. In the examplesdescribed herein, each of the first constituent predictor and the secondconstituent predictor is generated using intra or inter prediction asexplained above with regard to the encoder 400. At 604, the process 600generates a second predictor also using intra or inter prediction. At606, the process 600 combines the first compound predictor and thesecond predictor to obtain the prediction block. The process 600 isdescribed further with reference to the non-limiting examples of FIGS. 7and 9-12. In FIGS. 7 and 9-12, (REF_x, MV_x) indicates, respectively, areference frame and a motion vector of a corresponding inter predictor(i.e., an prediction block generated by an inter prediction operation).For example, inter predictor 706 of FIG. 7 is indicated as using (orreferring to) a reference frame REF_0 and a motion vector MV_0.Reference frames are described below with respect to FIG. 8.

FIG. 7 is a diagram 700 of an inter tri-prediction according toimplementations of this disclosure. FIG. 7 illustrates a two-levelcompound prediction that results in a prediction block at level two. Atlevel 0, indicated by line 720, the process 600 generates, at 602, atleast two predictor blocks, namely an inter predictor 704 and the interpredictor 706 in this example. The process 600 generates the interpredictor 704 using reference frame 716 of a reference frame buffer 702and generates the inter predictor 706 using the reference frame 714. Theprocess 600 combines the inter predictor 706 and the inter predictor 704to generate a first compound predictor block, namely a level-1 compoundpredictor 708.

At level 1, indicated by line 722, the process 600, via 604, generatesan inter predictor 710 (i.e., the second predictor). In this case, thesecond predictor is an inter predictor, i.e., is generated using interprediction. However, the second predictor can be generated using intraprediction. The process 600 generates the inter predictor 710 using areference frame 718 of the reference frame buffer 702. At 606, theprocess 600 combines the level-1 compound predictor 708 (i.e., the firstcompound predictor) and the inter predictor 710 (i.e., the secondpredictor) to obtain a level-2 compound predictor 712. The level-2compound predictor 712 constitutes the prediction block of the currentblock.

In some examples, the compound predictor at level 1 or level 2, or both,is generated as a weighted combination of pixels of the two separatepredictors. The two separate predictors can conform to the dimension ofthe current block such that the combination is performed on apixel-by-pixel basis for co-located pixels. The weighting of thepredictors can be determined in a variety of ways. For example, theweights of the two predictors can be, respectively, w0 and (1−w0) wherew0 is a value between zero and one. In some implementations, the weightw0 can be ½ and the weight (1−w0) can be ½, resulting in averaging ofthe pixels of each of the two predictors. Other values for the weight w0are possible. For example, the weight w0 can be a number between 0 and16. The weighting at the different levels can be the same or different.

As another example, wedge-based compound prediction can be used. Inwedge-based compound prediction, flexible weights can be adapted to thetextures and pixel positions of the current block. Wedge-based compoundprediction can partition the current block into two or more partitionsbased on one or more partition lines that can be straight or oblique(i.e., not straight). A codebook of available partition lines can beused. Wedge-based compound prediction can partition the current blockbased on the codebook of available partition lines. Each partition ofthe block can be separately predicted (i.e., can have a separatepredictor block) using inter and/or intra prediction. The separatepredictor blocks can be combined to form the compound predictor blockhaving the same dimensions as the current block being predictedaccording to the partition line(s).

In the case of one partition line, the current block is divided into twopartitions. Each partition of the current block can be separately interor intra predicted to generate a first predictor block and a secondpredictor block. The compound predictor can be a smooth-weighted or acliff-weighted combination of the first predictor block and the secondpredictor block.

A compound predictor that is a smooth-weighted compound predictor canhave weights that vary relatively smoothly across the compoundprediction block or can have constant weights (i.e., weights that do notvary). A cliff-weighted compound predictor block, in contrast, can varythe weights sharply from the first predictor block to the secondpredictor block with a transition region that can be narrow compared tothe size of the current block. The codebook can include one or moretransition regions and the weights to apply to the first predictor blockand the second predictor block.

FIG. 8 is a diagram of an example of a reference frame buffer 800according to implementations of this disclosure. As described above withrespect to inter prediction, a motion vector used to generate aprediction block refers to (i.e., uses) a reference frame. Referenceframes can be stored in a reference frame buffer, such as the referenceframe buffer 800.

A current frame, or a current block or region of a frame, can be encodedusing a reference frame such as a “last frame,” which is the adjacentframe immediately before the current frame in the video sequence. Whenvideo frames are encoded out of order (i.e., not in the sequence thatthey appear in the video stream), motion information from video framesin the past or future can be included in the candidate reference motionvectors. Encoding video frames can occur, for example, using so-called“alternate reference frames” that are not temporally neighboring to theframes coded immediately before or after them. An alternate referenceframe can be a synthesized frame that does not occur in the input videostream or is a duplicate frame to one in the input video stream that isused for prediction. An alternate frame may not be displayed followingdecoding. Such a frame can resemble a video frame in the non-adjacentfuture. Another example in which out of order encoding may occur isthrough the use of a so-called “golden frame,” which is a reconstructedvideo frame that may or may not be neighboring to a current frame and isstored in memory for use as a reference frame until replaced, e.g., by anew golden frame.

The reference frame buffer stores reference frames used to encode ordecode blocks of frames of a video sequence. The reference frame buffercan include reference frames such as those described above. For example,the reference frame buffer 800 can include a last frame LAST_FRAME 802,a golden frame GOLDEN_FRAME 804, and an alternative reference frameALTREF_FRAME 806. A reference frame buffer can include more, fewer, orother reference frames. The reference frame buffer 800 is shown asincluding eight reference frames. However, a reference frame buffer caninclude more or fewer than eight reference frames.

The last frame LAST_FRAME 802 can be, for example, the adjacent frameimmediately before the current frame in the video sequence. The goldenframe GOLDEN_FRAME 804 can be, for example, a reconstructed video framefor use as a reference frame that may or may not be adjacent to thecurrent frame. The alternative reference frame ALTREF_FRAME 806 can be,for example, a video frame in the non-adjacent future, which is abackward reference frame.

The reference frames stored in the reference frame buffer 800 can beused to identify motion vectors for predicting blocks of frames to beencoded or decoded. Different reference frames may be used depending onthe type of prediction used to predict a current block of a currentframe. For example, when compound prediction is used, multiple frames,such as one for forward prediction (e.g., LAST_FRAME 802 or GOLDEN_FRAME804) and one for backward prediction (e.g., ALTREF_FRAME 806) can beused for predicting the current block.

There may be a finite number of reference frames that can be storedwithin the reference frame buffer 800. As shown in FIG. 8, the referenceframe buffer 800 can store up to eight reference frames. Although threeof the eight spaces in the reference frame buffer 800 are used by theLAST_FRAME 802, the GOLDEN_FRAME 804, and the ALTREF_FRAME 806, fivespaces remain available to store other reference frames.

In particular, one or more available spaces in the reference framebuffer 800 may be used to store a second last frame LAST2_FRAME and/or athird last frame LAST3_FRAME as additional forward reference frames, inaddition to the LAST_FRAME 802. A backward frame BWDREF_FRAME 808 can bestored as an additional backward prediction reference frame, in additionto ALTREF_FRAME 806. The BWDREF_FRAME can be closer in relative distanceto the current frame than the ALTREF_FRAME 806, for example.

In one example, the pair of {LAST_FRAME, BWDREF_FRAME} can be used togenerate a compound predictor for coding the current block. In thisexample, LAST_FRAME is a “nearest” forward reference frame for forwardprediction, and BWDREF_FRAME is a “nearest” backward reference frame forbackward prediction.

A current block is predicted based on a prediction mode. The predictionmode may be selected from one of multiple intra-prediction modes. In thecase of inter prediction, the prediction mode may be selected from oneof multiple inter-prediction modes using one or more reference frames ofthe reference frame buffer 800 including, for example, the LAST_FRAME802, the GOLDEN_FRAME 804, the ALTREF_FRAME 806, or any other referenceframe. The prediction mode of the current block can be transmitted froman encoder, such as the encoder 400 of FIG. 4, to a decoder, such as thedecoder 500 of FIG. 5, in an encoded bitstream, such as the compressedbitstream 420 of FIGS. 4-5. A bitstream syntax can support threecategories of inter prediction modes in an example. These interprediction modes can include a mode (referred to herein as the ZERO_MVmode) in which a block from the same location within a reference frameas the current block is used as the prediction block, a mode (referredto herein as the NEW_MV mode) in which a motion vector is transmitted toindicate the location of a block within a reference frame to be used asthe prediction block relative to the current block, or a mode (referredto herein as the REF_MV mode and comprising a NEAR_MV or NEAREST_MVmode) in which no motion vector is transmitted and the current blockuses the last or second-to-last non-zero motion vector used byneighboring, previously coded blocks to generate the prediction block.The previously coded blocks may be those coded in the scan order, e.g.,a raster or other scan order, before the current block. Inter-predictionmodes may be used with any of the available reference frames. NEAREST_MVand NEAR_MV can refer to the most and second most likely motion vectorsfor the current block obtained by a survey of motion vectors in thecontext for a reference. The reference can be a causal neighborhood incurrent frame. The reference can be co-located motion vectors in theprevious frame.

FIG. 9 is a diagram 900 of an inter-intra tri-prediction according toimplementations of this disclosure. FIG. 9 illustrates another exampleof a two-level compound prediction. At level 0, indicated by line 920,the process 600 generates, at 602, at least two predictor blocks, namelyan intra predictor 904 and an inter predictor 906 in this example. Theprocess 600 generates the inter predictor 906 using a reference frame916 of a reference frame buffer 902. The reference frame buffer 902 canbe the reference frame buffer 800 of FIG. 8. The process 600 combinesthe inter predictor 906 and the intra predictor 904 to generate a firstcompound predictor, namely a level-1 compound predictor 908.

At level 1, indicated by line 922, the process 600, at 604, generates aninter predictor 910 (i.e., the second predictor). In this example, likethat in FIG. 7, the second predictor is an inter predictor. However, thesecond predictor can be an intra predictor. The process 600 generatesthe inter predictor 910 using a reference frame 918 of the referenceframe buffer 902. At 606, the process 600 combines the level-1 compoundpredictor 908 (i.e., the first compound predictor) and the interpredictor 910 (i.e., the second predictor) to obtain a level-2 compoundpredictor 912. The level-2 compound predictor 912 constitutes theprediction block of the current block.

The process 600 can combine the inter predictor 906 and intra predictor904 as described with respect to FIG. 7. In the illustrated example, theweights w0_0, w0_1 are used for the inter predictor 906 and intrapredictor 904, respectively. The process 600 can combine the level-1compound predictor 908 and the inter predictor 910 as described withrespect to FIG. 7. In the illustrated example, the weights w1_0, w1_1are used for the level-1 compound predictor 908 and the inter predictor910, respectively. The weights w0_0, w0_1 can be the same or differentfrom the w1_0, w1_1. Note that a different technique may be used tocombine the predictors at level 0 from that used to combine thepredictors at level 1 in this or the other embodiments.

FIG. 10 is a diagram 1000 of an inter quad-prediction according toimplementations of this disclosure. FIG. 10 illustrates another exampleof a two-level compound prediction. At level 0, indicated by line 1016,the process 600 generates, at 602, at least two predictor blocks, namelyan inter predictor 1002 and an inter predictor 1004. The process 600generates the inter predictor 1002 using reference frame 1020 of areference frame buffer 1028 and generates the inter predictor 1004 usingthe reference frame 1022. The reference frame buffer 1028 can be thereference frame buffer 800 of FIG. 8. The process 600 combines (e.g.,using weights 1/2 and 1/2) the inter predictor 1002 and the interpredictor 1004 to generate a first compound predictor, namely a level-1compound predictor_0 1010.

At 604, the process 600 generates a second predictor block. In FIG. 10,the process 600 generates the second compound predictor by combining atleast two other predictor blocks for the current block. Namely, thesecond predictor is a level-1 compound predictor-1 1012, which is acombination (e.g., using weights 1/2 and 1/2) of an inter predictor 1006(generated using a reference frame 1024) and an inter predictor 1008(generated using a reference frame 1026). Alternatively, one or both ofthe same predictor blocks may be used. They may be combined using adifferent technique to generate the level-1 compound predictor_1 1012.

At 606, the process 600 combines (e.g., using weights 1/2 and 1/2) thelevel-1 compound predictor_0 1010 (i.e., the first compound predictor)and the level-1 compound predictor_1 1012 (i.e., the second predictor)to obtain a level-2 compound predictor 1014. The level-2 compoundpredictor 1014 constitutes the prediction block of the current block.

FIG. 11 is a diagram 1100 of an inter-intra quad-prediction according toimplementations of this disclosure. FIG. 11 illustrates another exampleof a two-level compound prediction. At level 0, indicated by line 1116,the process 600 generates, at 602, at least two predictor blocks, namelyan inter predictor 1102 and an intra predictor 1104. The process 600generates the inter predictor 1102 using reference frame 1120 of areference frame buffer 1128 in this example. The reference frame buffer1128 can be the reference frame buffer 800 of FIG. 8. The process 600combines (e.g., using weights W0_0 and W0_1) the inter predictor 1102and the intra predictor 1104 to generate a first compound predictorblock, namely a level-1 compound predictor_0 1110.

At 604, the process 600 generates a second predictor block. In FIG. 11,the second predictor block is a compound predictor; namely, the secondpredictor is a level-1 compound predictor-1 1112, which is a combination(e.g., using the weights W0_2 and W0_3), of at least two other predictorblocks, an inter predictor 1106 (generated using a reference frame 1124)and an intra predictor 1108. Alternatively, one or both of the samepredictor blocks may be used. They may be combined using a differenttechnique to generate the level-1 compound predictor_1 1112. The weightsW0_0 and W0_1 and the weights W0_2 and W0_3 may be the same ordifferent.

At 606, the process 600 combines the level-1 compound predictor_0 1110(i.e., the first compound predictor) and the level-1 compoundpredictor_1 1112 (i.e., the second predictor) to obtain a level-2compound predictor 1114. The level-2 compound predictor 1114 constitutesthe prediction block of the current block. In this example, weights W1_0and W1_1 are used, which may be the same or different from the weightsW0_0 and W0_1 and/or the weights W0_2 and W0_3.

FIG. 12 is a diagram 1200 of a multi-level prediction according toimplementations of this disclosure. FIG. 12 illustrates an example of amulti-level compound prediction. In this example, a level-3 compoundpredictor 1214 is generated at level-3. The level-3 compound predictor1214 constitutes the prediction block of the current block At level 0,indicated by line 1216, the process 600 generates, at 602, at least twopredictor blocks, namely an intra predictor 1206 and an intra predictor1207, which are combined to form a first compound predictor, namely alevel-1 compound predictor_3 1212.

As shown in the example of FIG. 12, the process 600 can also create, atlevel 1, a level-1 compound predictor_1 1210 (by combining an interpredictor 1202 and an intra predictor 1203) and a level-1 compoundpredictor_2 1211 (by combining an inter predictor 1204 and an intrapredictor 1205). The level-1 compound predictor 1210 and the level-1compound predictor-2 are combined to generate, at level 2 indicated byline 1220, a level-2 compound predictor 1213.

As the level-1 compound predictor_3 1212 is not combined with any otherlevel 1 predictor block (compound or otherwise), it can be carriedforward to level 2.Alternatively, the process 600 can generate thelevel-1 compound predictor_3 1212 at level 2 instead of at level 1.

At 604, the process 600 can generate any number of second predictorsdepending on a number of desired levels of compound prediction. Forexample, the level-1 compound predictor_1 1210, the level-1 compoundpredictor_2 1211, the level-1 compound predictor_3 1212, and the level-2compound predictor 1213 can be second predictors.

At 606, the process 600 obtains the level-3 compound predictor 1214. Asillustrated with respect to the diagram 1200, combining the firstcompound predictor and the second predictor can be accomplished viamultiple intermediary levels.

Thus, the process 600 can create the prediction block of the currentblock in multiple levels. At level 0, one or more intra predictorsand/or inter predictors are created. The level-0 predictors (e.g., theinter predictor 704 and the inter predictor 706 of FIG. 7) can becombined as described above to generate one or more level-1 compoundpredictors (e.g., the level-1 compound predictor 708 of FIG. 7). Some ofthe level-1 compound predictors (e.g., the level-1 compound predictor_01010 and level-1 compound predictor_1 of FIG. 10) can be combined tocreate one or more level-2 compound predictors (e.g., the level-2compound predictor 1014 of FIG. 10). Depending on the number of desiredlevels, additional inter and/or intra predictors can be generated at alevel-x (where x is greater than 0) (e.g., the inter predictor 710 ofFIG. 7 and the inter predictor 910 of FIG. 9). These additional interand/or intra predictors can be combined with the level-x compoundpredictors to generate level-(x+1) compound predictors (e.g., combiningthe level-1 compound predictor 708 and the inter predictor 710 to formthe level-2 compound predictor 712 of FIG. 7).

While specific inter and intra predictors are shown in FIGS. 7 and 9-12at the different levels, the disclosure is not so limited. Any inter,intra, or combinations of inter and intra predictors can be used at anylevel. For example, the inter predictor 706 of FIG. 7 can be an intrapredictor, the inter predictor 910 of FIG. 9 can be an inter predictor,the intra predictor 1108 of FIG. 11 can be an inter predictor, and soon. Additionally, while compound predictors that combine two predictorsare illustrated, the disclosure is not so limited. A compound predictorcan be obtained by combining any number of intra, inter, or compoundpredictors.

Additional ways of combining predictor blocks can be available. Asdisclosed herein or otherwise, various ways of combining predictorblocks (e.g., intra or inter predictor blocks such as the interpredictor 704 and the inter predictor 706) to generate a compoundpredictor, a compound predictor with another compound predictor (e.g.,level-1 compound predictor_0 1010 with level-1 compound predictor-1 1012of FIG. 10, or level-1 compound predictor_1 1210 with level-1 compoundpredictor_2 1211 of FIG. 12), a compound predictor with an intra or aninter predictor (e.g., the level-1 compound predictor 708 and the interpredictor 710 of FIG. 7), or any combination thereof, may be available.

Referring again to FIG. 7 as an illustrative example, implementations ofdetermining the predictors for the multi-level compound prediction isdescribed. The level-1 compound predictor 708 can be obtained byevaluating (i.e., searching for) motion vectors for pairs of referenceframes until a best set of motion vectors of a pair of reference framesis obtained. All, or less than all, possible pairs of reference framescan be evaluated (i.e., searched). The one or more best motion vectorsthat form the set of motion vectors correspond to the bestrate-distortion value. A rate-distortion value refers to a ratio thatbalances an amount of distortion (i.e., loss in video quality) with rate(i.e., the number of bits) used for encoding. In FIG. 7, the referenceframes corresponding to the best motion vectors MV_0, MV_1 aredetermined to be reference frames 714 and 716.

The above search process can also identify, for the current block,motion vectors for the NEAREST_MV and NEAR_MV modes for at least some ofthe reference frames of the reference frame buffer 702. For eachsearched reference frame a best motion vector can be identified. Themotion vectors for NEAREST_MV and NEAR_MV, and single motion vectors,can be added to the reference motion vector list of the current block.

For levels below level 0, a new motion search can be performed todetermine the best motion vector and reference frame to generate thesecond predictor (e.g., inter predictor 710). Alternatively, the bestsingle motion vector from the reference motion vector list of thecurrent block (e.g., based on a lowest rate distortion value) can beused to generate the inter predictor 710. In FIG. 7, the reference frame714 is the best single motion vector and can be used (as indicated bydashed line 726) to generate the inter predictor 710. In someembodiments, a per se rule may be established that the NEAREST_MV orNEAR_MV inter-prediction mode is used to generate an inter predictor fora next level, reducing the number of bits used to encode the motionvector information.

With either alternative (i.e., using the single motion vector or usingNEAREST_MV or NEAR_MV), no new motion search is performed to generatethe second predictor thereby reducing complexity. As such, informationalready identified (i.e., generated), while generating the level-1compound predictor 708, can be used to generate multi-level compoundpredictors.

The process 600 can encode, in an encoded bitstream such as thecompressed bitstream 420 of FIGS. 4 and 5, an indicator (e.g., one ormore syntax elements) of a compound prediction type. A decoder, such asthe decoder 500 of FIG. 5, can decode the indicator to determine, forexample, the number of reference frames and motion frames to decode fromthe encoded bitstream in order for the decoder to perform multi-levelcompound prediction that can be identical to the motion prediction ofthe encoder. For example, the indicator can identify a tri-prediction ora quad-prediction compound prediction type. An indicator ofquad-prediction can identify that the second predictor is a compoundpredictor. In another example, an indicator of tri-prediction canidentify that the second predictor is not a compound predictor.

In either event, the indicator can be accompanied by syntax elements,encoded into the bitstream by the process 600, or after the process 600is completed, that indicate the number of levels of multi-level compoundprediction that the decoder is to perform and the prediction modes ofeach of the predictors. For example, and referring to FIG. 12, theencoder can indicate, via syntax elements, three levels of compoundprediction, and that, at level 1, three compound predictors are to begenerated: the first is an inter-intra compound predictor, the second isan inter-intra compound predictor, and the third is an intra-intracompound predictor. The encoder can encode additional information thatcan be used by the decoder. Such information can include identifiers ofintra-prediction modes and/or identifiers of motion vectors andinter-prediction reference frames for the predictors forming theprediction block used to encode the current block.

Multi-level compound prediction can result in encoding additional bitsin the encoded bitstream to generate the prediction block as compared toexisting techniques. This can occur due to the need to encode syntaxelements identifying multiple reference frames, multiple motion vectorsand/or multiple intra prediction modes. The additional bits can outweighthe compression benefits of using multi-level compound prediction.Implementations of multi-level compound prediction can include steps ortechniques that reduce the bitrate overhead while maintaining areasonable level of encoder and decoder complexity.

In an example of reducing the bitrate overhead, performing and coding anew motion vector for an inter predictor at level-x (where x>0), boththe encoder and the decoder can use the REF_MV mode of a reference frameto obtain an inter predictor. Further syntax can specify whetherNEAREST_MV or NEAR_MV is to be used for inter prediction. As such, nomotion vector need be written to the encoded bitstream for interpredictions in level-x (where x>0).

In another example of reducing the bitrate overhead, all interpredictions in the multi-level compound prediction may use a singleinter prediction mode. For example, a special mode can be signaled bythe encoder that indicates that all inter predictions in the multi-levelcompound prediction (e.g., when the indicator for multi-level compoundprediction is present) use the NEAREST_MV mode.

Reducing the bitrate overhead may also be aided by coding the additionalmotion vectors more effectively. The motion vectors of the firstcompound predictor (e.g., the motion vectors corresponding to the interpredictors 704 and 706 of FIG. 7 or the motion vectors corresponding tothe inter predictors 1002 and 1004 of FIG. 10) are known before the timeof encoding of, e.g., a third or fourth motion vector for themulti-level compound prediction (e.g., the motion vector correspondingto the inter predictor 710 of FIG. 7 or the motion vectors correspondingto the inter predictors 1006 and 1008 of FIG. 10). As alluded to above,the motion vector(s) of the first compound predictor can be added to thereference motion vector list of candidate reference motion vectors.These motion vectors may provide a better motion vector reference forencoding the third and/or fourth motion vector(s) than NEAREST_MV orNEAR_MV. Where such a motion vector provides a better motion vectorreference for encoding, more efficient encoding of the third and/orfourth motion vector(s) can result.

The coding of the reference frames can be more efficient, reducing thebitrate overhead, by improving the context used. In entropy coding(e.g., at the entropy encoding stage 408 or the entropy decoding stage502), a context model may be used (also referred to as probabilitycontext model or probability model) that provides estimates ofconditional probabilities for coding the symbols representing thetransform coefficient. Previously coded data can be used a context forselecting the context model to encoding the current block. For example,an encoder can apply a specific context when a third reference frame isidentical to either of the two reference frames of the first compoundpredictor. As another example, the encoder can signal a specific modebased on the reference frames used by the first compound predictor andthe third and/or fourth reference frame corresponding to the thirdand/or fourth motion vector. For example, when the reference frame pairof the first compound predictor is a special pattern, e.g. {LAST_FRAME,BWDREF_FRAME}, and the third reference frame is LAST_FRAME, the encodercan so signal via syntax elements and no identifier of a reference needbe encoded.

FIG. 13 is a flowchart diagram of a process 1300 for decoding a currentblock using multi-level compound prediction according to implementationsof this disclosure. The process 1300 may be performed by a decoder. Forexample, the process 1300 can be performed in whole or in part by theintra/inter-prediction stage 508 of the decoder 500. The process 1300can be performed in whole or in part during the reconstruction path(shown by the dotted connection lines) of the encoder 400 of FIG. 4.Implementations of the process 1300 can be performed by storinginstructions in a memory such as the memory 204 of the receiving station106, or the transmitting station 102, to be executed by a processor suchas CPU 202, for example.

The process 1300 can be implemented using specialized hardware orfirmware. Some computing devices can have multiple memories, multipleprocessors, or both. The steps or operations of the process 1300 can bedistributed using different processors, memories, or both. Forsimplicity of explanation, the process 1300 is depicted and described asa series of steps or operations. However, the teachings in accordancewith this disclosure can occur in various orders and/or concurrently.Additionally, steps in accordance with this disclosure may occur withother steps not presented and described herein. Furthermore, not allillustrated steps or operations may be used to implement a method inaccordance with the disclosed subject matter.

At 1302, the process 1300 decodes, from an encoded bitstream, anindicator that identifies that multi-level compound prediction was usedto encode the current block. The indicator can be one or more syntaxelements that directs the decoder to generate a prediction block for thecurrent block using multi-level compound prediction. The indicator maybe decoded from a frame header, a slice header, or a block header, forexample. In some cases, the indicator may comprise syntax split betweenheaders. For example, a frame level indicator may identify that at leastsome blocks in the frame using multi-level compound prediction, while ablock level indicator identifies the type of multi-level compoundprediction, along with the inter-prediction and/or intra-predictionmodes used for the constituent predictor blocks of the current block.Motion vector and reference frame information may also be included.

At 1304, the process 1300 generates a first compound predictor for thecurrent block by combining at least two predictor blocks for the currentblock. For example, the indicator can include syntax that identifies oneor more reference frames and corresponding motion vectors, one or moreintra-prediction modes, or any combination thereof, for each of thepredictor blocks. Based on this information, the process 1330 cangenerate the first compound predictor such as the level-1 compoundpredictor 708 of FIG. 7, the level-1 compound predictor 908 of FIG. 9,the level-1 compound predictor_0 1010 of FIG. 10, the level-1 compoundpredictor_0 1110 of FIG. 11, and the like, by first generating theconstituent predictors and combining them to form the first compoundpredictor based on the combination technique (such as the weights) usedby the encoder. The combination technique can be decided a prioribetween the encoder and decoder, or can be signaled from the encoder tobe decoded and used by the decoder.

At 1306, the process 1300 generates a second predictor for the currentblock. For example, the indicator may identify an intra-prediction modeor an inter-prediction mode for the second predictor. The process 1300generates the second predictor using the identified prediction mode. Theindicator may identify that the second predictor is inter predicted bythe inclusion of a reference frame and/or a motion vector in the encodedbitstream.

In some cases, the indicator identifies that the second predictor is acompound predictor. That is, the second predictor is generated bycombining at least two predictor blocks. If so, the process 1300 at 1306can determine, from the encoded bitstream, the inter-prediction mode(s)and/or inter-prediction modes used, and generate the constituentpredictor blocks before combining them to form the second predictor. Thecombining may be performed by an a priori decision between the encoderand decoder, by information signaled from the encoder to the decoder.

In some implementations, the inclusion of the indicator in the encodedbitstream can indicate to use NEAREST_MV for every inter-predictedpredictor block in one or more levels. As such, the decoder does notreceive a separate motion vector coding mode for those blocks.Additionally or alternatively, an a priori rule may be implemented thatallows the decoder to determine the reference frame for a level-2predictor based on one or more reference frames used for a level-1predictor.

At 1308, the process 1300 combines the first compound predictor and thesecond predictor to obtain a prediction block for decoding the currentblock. The combination is made the same way as was done at the encoder.

The aspects of encoding and decoding described above illustrate someencoding and decoding techniques. However, it is to be understood thatencoding and decoding, as those terms are used in the claims, could meancompression, decompression, transformation, or any other processing orchange of data.

The words “example” or “implementation” are used herein to mean servingas an example, instance, or illustration. Any aspect or design describedherein as “example” or “implementation” is not necessarily to beconstrued as preferred or advantageous over other aspects or designs.Rather, use of the words “example” or “implementation” is intended topresent concepts in a concrete fashion. As used in this application, theterm “or” is intended to mean an inclusive “or” rather than an exclusive“or”. That is, unless specified otherwise, or clear from context, “Xincludes A or B” is intended to mean any of the natural inclusivepermutations. That is, if X includes A; X includes B; or X includes bothA and B, then “X includes A or B” is satisfied under any of theforegoing instances. In addition, the articles “a” and “an” as used inthis application and the appended claims should generally be construedto mean “one or more” unless specified otherwise or clear from contextto be directed to a singular form. Moreover, use of the term “animplementation” or “one implementation” throughout is not intended tomean the same embodiment or implementation unless described as such.

Implementations of transmitting station 102 and/or receiving station 106(and the algorithms, methods, instructions, etc., stored thereon and/orexecuted thereby, including by encoder 400 and decoder 500) can berealized in hardware, software, or any combination thereof. The hardwarecan include, for example, computers, intellectual property (IP) cores,application-specific integrated circuits (ASICs), programmable logicarrays, optical processors, programmable logic controllers, microcode,microcontrollers, servers, microprocessors, digital signal processors orany other suitable circuit. In the claims, the term “processor” shouldbe understood as encompassing any of the foregoing hardware, eithersingly or in combination. The terms “signal” and “data” are usedinterchangeably. Further, portions of transmitting station 102 andreceiving station 106 do not necessarily have to be implemented in thesame manner.

Further, in one aspect, for example, transmitting station 102 orreceiving station 106 can be implemented using a computer or processorwith a computer program that, when executed, carries out any of therespective methods, algorithms and/or instructions described herein. Inaddition, or alternatively, for example, a special purposecomputer/processor can be utilized which can contain other hardware forcarrying out any of the methods, algorithms, or instructions describedherein.

Transmitting station 102 and receiving station 106 can, for example, beimplemented on computers in a video conferencing system. Alternatively,transmitting station 102 can be implemented on a server and receivingstation 106 can be implemented on a device separate from the server,such as a hand-held communications device. In this instance,transmitting station 102 can encode content using an encoder 400 into anencoded video signal and transmit the encoded video signal to thecommunications device. In turn, the communications device can thendecode the encoded video signal using a decoder 500. Alternatively, thecommunications device can decode content stored locally on thecommunications device, for example, content that was not transmitted bytransmitting station 102. Other transmitting station 102 and receivingstation 106 implementation schemes are available. For example, receivingstation 106 can be a generally stationary personal computer rather thana portable communications device and/or a device including an encoder400 may also include a decoder 500.

Further, all or a portion of implementations of the present disclosurecan take the form of a computer program product accessible from, forexample, a tangible computer-usable or computer-readable medium. Acomputer-usable or computer-readable medium can be any device that can,for example, tangibly contain, store, communicate, or transport theprogram for use by or in connection with any processor. The medium canbe, for example, an electronic, magnetic, optical, electromagnetic, or asemiconductor device. Other suitable mediums are also available.

The above-described embodiments, implementations and aspects have beendescribed in order to allow easy understanding of the present disclosureand do not limit the present disclosure. On the contrary, the disclosureis intended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims, which scope is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structure as is permitted under the law.

What is claimed is:
 1. A method for coding a current block, comprising:generating a first compound predictor by combining at least twopredictor blocks for the current block; generating a second predictorfor the current block; and combining the first compound predictor andthe second predictor to obtain a prediction block for coding the currentblock.
 2. The method of claim 1, wherein the second predictor is asecond compound predictor and generating the second predictor comprises:generating the second compound predictor by combining at least two otherpredictor blocks for the current block.
 3. The method of claim 1,wherein generating the second predictor comprises: generating the secondpredictor using inter prediction.
 4. The method of claim 3, whereingenerating the second predictor comprises: using NEAREST_MV to interpredict the current block.
 5. The method of claim 1, wherein generatingthe second predictor comprises: generating the second predictor usingintra prediction.
 6. The method of claim 1, wherein the at least twopredictor blocks comprises a first constituent predictor and a secondconstituent predictor, generating the first compound predictor comprisesaveraging pixels of the first constituent predictor with the secondconstituent predictor, and combining the first compound predictor andthe second predictor comprises averaging pixels of the first compoundpredictor and the second predictor to obtain the prediction block. 7.The method of claim 1, further comprising: encoding, in an encodedbitstream, an indicator identifying whether the second predictor is acompound predictor.
 8. The method of claim 1, further comprising:generating each of the at least two predictor blocks using interprediction.
 9. An apparatus for encoding a current block, the apparatuscomprising: a memory; and a processor configured to execute instructionsstored in the memory to: generate a first compound predictor bycombining at least two predictor blocks for the current block; generatea second predictor for the current block; and combine the first compoundpredictor and the second predictor to obtain a prediction block forencoding the current block.
 10. The apparatus of claim 9, wherein theinstructions to generate the second predictor further comprisesinstructions to: generate a second compound predictor by combining atleast two other predictor blocks for the current block.
 11. Theapparatus of claim 10, wherein the at least two other predictor blockscomprise a first constituent predictor generated using intra predictionand a second constituent predictor generated using inter prediction. 12.The apparatus of claim 9, wherein the instructions to generate thesecond predictor comprise instructions to: generate the second predictorby inter predicting the current block using NEAREST_MV.
 13. Theapparatus of claim 9, wherein the second predictor is an intrapredictor.
 14. The apparatus of claim 9, wherein the at least twopredictors comprise a first constituent predictor and a secondconstituent predictor, the first constituent predictor is generatedusing a first adjacent frame as a reference frame in inter prediction ofthe current block and the second constituent predictor is generatedusing a second adjacent frame in inter prediction of the current block,the first adjacent frame immediately before a current frame in which thecurrent block is located in a video sequence and the second adjacentframe immediately before the first adjacent frame in the video sequence,and the second predictor is generated using the first adjacent frame asa reference frame in inter prediction of the current block.
 15. Anapparatus for decoding a current block, the apparatus comprising: amemory; and a processor configured to execute instructions stored in thememory to: decode, from an encoded bitstream, an indicator thatidentifies that multi-level compound prediction was used to encode thecurrent block; generate a first compound predictor by combining at leasttwo predictor blocks for the current block; generate a second predictorfor the current block; and combine the first compound predictor and thesecond predictor to obtain a prediction block for decoding the currentblock.
 16. The apparatus of claim 15, wherein the indicator identifieswhether or not the second predictor is a compound predictor block. 17.The apparatus of claim 15, wherein the instructions to combine the firstcompound predictor and the second predictor comprise instructions toaverage co-located pixels of the first compound predictor and the secondpredictor to obtain the prediction block.
 18. The apparatus of claim 15,wherein the indicator identifies a prediction mode for each of the atleast two predictor blocks.
 19. The apparatus of claim 18, wherein theat least two predictor blocks comprise a first inter predictor and asecond inter predictor, and wherein the instructions compriseinstructions to use a same inter-prediction mode for each of the firstinter predictor, the second inter predictor, and the second predictor.20. The apparatus of claim 15, wherein the instructions to generate thefirst compound predictor comprise instructions to average co-locatedpixels of the at least two predictor blocks to generate the firstcompound predictor.